The present invention relates to detection of a leak or breach in a fuel storage tank and/or in the interstitial space of a storage tank, and particularly for fuel storage tanks used to hold fuel in retail service station environments.
In service station environments, fuel is delivered to fuel dispensers from fuel storage tanks. The fuel storage tanks are large containers located beneath the ground that contain fuel. A separate fuel storage tank is provided for each fuel type, such as low octane gasoline, high-octane gasoline, and diesel. In order to deliver the fuel from the fuel storage tanks to the fuel dispensers, a submersible turbine pump is provided that pumps the fuel out of the fuel storage tank and delivers the fuel through a main fuel piping conduit that runs beneath the ground in the service station.
Due to regulatory requirements governing service stations, fuel storage tanks are required to be encased in a second or outer casing such that the fuel storage tank contains two walls. These tanks are sometimes referred to as xe2x80x9cdouble-walled tanks.xe2x80x9d A double-walled tank is comprised of an inner vessel that holds liquid fuel surrounded by an outer casing. An annular space, also called an xe2x80x9cinterstitial space,xe2x80x9d is formed between the inner vessel and the outer casing. Any leaked fuel that occurs due to a breach of the inner vessel is captured inside the interstitial space instead of leaking to the ground so long as there are no breaches in the outer casing. The outer casing of the fuel storage tank serves as an extra measure of protection to prevent leaked fuel from reaching the ground. An example of double-walled fuel storage tank is disclosed in U.S. Pat. No. 5,115,936, incorporated herein by reference in its entirety.
It is possible that the outer casing of the double-walled fuel storage tank could contain a leak or breach. In this case, if fuel leaks out of the inner vessel into the interstitial space, this fuel may escape to the ground through breach in the outer casing. Therefore, it is desirable to determine if there is a breach or leak in the outer casing of the fuel storage tank as soon as possible before a fuel leak occurs so that such breach can be alleviated before any leaked fuel from the inner vessel could reach the ground.
Prior known leak detection systems are described in U.S. Pat. Nos. 4,676,093 and 4,672,366. These patents disclose a xe2x80x9cdryxe2x80x9d and xe2x80x9cwetxe2x80x9d leak detection systems that both have drawbacks. The xe2x80x9cdryxe2x80x9d system consists of placing detectors sensitive to the presence of fluid in the interstitial space of the fuel storage tank. A sensor detects a leak in the interstitial space, but this leak would reach the ground if a leak also existed in the outer casing of the fuel storage tank since a breach in the outer casing is not detected in this system.
In the xe2x80x9cwetxe2x80x9d system, the interstitial space is filled with a liquid, such as ethylene glycol, water, or brine solution. When either the inner vessel or the outer casing of the fuel storage tank is punctured or otherwise develops a leak, at least a portion of the liquid contained in the interstitial space will flow through such leak resulting in a reduction of volume of the solution. However, these systems only detect a leak when the leak has already occurred into the environment.
Another leak detection system that incorporates pressure monitoring is described in U.S. Pat. No. 3,848,765. This patent describes monitoring the pressure in the interstitial space of the fuel storage tank as a method of determining if a breach exists. If a certain amount of pressure decay occurs, this is indicative of a breach or leak in the outer casing of the fuel storage tank that will result in a leak of fuel to the environment should the inner wall of the fuel storage tank develop a leak. This system has the advantage of possibly detecting a breach in the outer casing of the fuel storage tank before a leak occurs so that preventive measures and alarms can be generated before any leaked fuel reaches the environment. However, a major drawback of this system is that it requires a vacuum generator to pressurize the interstitial space so that pressure decay in the interstitial space, if any, can be monitored. However, providing a vacuum generator to pressurize the interstitial space adds substantial costs in both the cost of the vacuum generator and its installation and maintenance costs thereby making such a system extremely cost prohibitive.
The present invention involves use of vacuum level monitoring of the interstitial space of a double-walled fuel storage tank to determine if a breach or leak exists in the outer casing of the tank since this technique has the advantage of detecting a breach possibly before a leak actually occurs. However, the present invention, unlike previous pressure monitoring systems, eliminates the extra cost of an additional vacuum generator to pressurize the interstitial space thereby making this system much more feasible to deploy.
The present invention relates to a sensing unit and tank monitor that monitors the vacuum level in the interstitial space of a double-walled fuel storage tank to determine if a breach or leak exist in the outer casing of the fuel storage tank. If the interstitial space cannot maintain a vacuum level and over a given amount of time after being pressurized, this is indicative that the outer casing of the fuel storage tank contains a breach or leak. If the inner vessel of the fuel storage tank were to incur a breach or leak such that fuel reaches the interstitial space of the fuel storage tank, this same fuel would also have the potential to reach the ground through the breach in the outer casing.
A sensing unit is provided that is communicatively coupled to a tank monitor or other control system. The sensing unit contains a pressure sensor that is coupled to vacuum tubing. The vacuum tubing is coupled to the interstitial space of the fuel storage tank, and is also coupled to a submersible turbine pump (STP) so that the STP can be used as a vacuum source to generate a vacuum level in the vacuum tubing and the interstitial space. The sensing unit and/or tank monitor determines if there is a leak or breach in the interstitial space by generating a vacuum in the interstitial space using the STP and subsequently monitoring the interstitial space using a pressure sensor to determine if the vacuum level changes significantly to indicate a leak. The system checks for both catastrophic and precision leaks.
In one leak detection embodiment of the present invention, the STP provides a vacuum source to the vacuum tubing and the interstitial space of the fuel storage tank. The tank monitor receives the vacuum level of the interstitial space via the measurements from the pressure sensor and the sensing unit. After the vacuum level in the interstitial space reaches a defined initial threshold vacuum level, the STP is deactivated and isolated from the interstitial space. The vacuum level of the interstitial space is monitored. If the vacuum level decays to a catastrophic threshold vacuum level, the STP is activated to restore the vacuum level. If the STP cannot restore the vacuum level to the defined initial threshold vacuum level in a defined amount of time, a catastrophic leak detection alarm is generated and the STP is shut down.
If the vacuum level in the interstitial space is restored to the defined initial threshold vacuum level within a defined period of time, a precision leak detection test is performed. The sensing unit monitors the vacuum level in the interstitial space to determine if the vacuum level decays to a precision threshold vacuum level within a defined period of time, in which case a precision leak detection alarm is generated, and the STP may be shut down.
Once a catastrophic leak or precision leak detection alarm is generated, service personnel are typically dispatched to determine if a leak really exists, and if so, to take corrective measures. Tests are conducted to determine if the leak exists in the vacuum tubing in the sensing unit or in the interstitial space.
The sensing unit also contains a liquid trap conduit. A liquid detection sensor is placed inside the liquid trap conduit so that any liquid that leaks in the sensing unit are reported. The sensing unit and tank monitor can detect liquid in the sensing unit at certain times or at all times If a liquid leak is detected by the tank monitor, the tank monitor will shut down the STP if so programmed.
Functional tests may also be performed to determine if the vacuum leak detection and liquid leak detection systems of the present invention are functioning properly. For the functional vacuum leak detection test, a leak is introduced into the interstitial space. A vacuum leak detection alarm not being generated by the sensing unit and/or the tank monitor is indicative that some component of the vacuum leak detection system is not working properly.
A functional liquid leak detection test can be also used to determine if the liquid detection system is operating properly. The liquid detection sensor is removed from the liquid trap conduit and submerged into a container of liquid or a purposeful liquid leak is injected into the liquid trap conduit to determine if a liquid leak detection alarm is generated. A liquid leak detection alarm not being generated by the sensing unit and/or the tank monitor is indicative that there has been a failure or malfunction with the liquid detection system.
The tank monitor may be communicatively coupled to a site controller and/or remote system to communicate leak detection alarms and other information obtained by the sensing unit. The site controller may pass information from the tank monitor onward to a remote system, and the tank monitor may communicate such information directly to a remote system.
Those skilled in the art will appreciate the scope of the present invention and realize additional aspects thereof after reading the following detailed description of the invention in association with the accompanying drawing figures.